1. Field of the Invention
This invention relates to insulated pipe strings for use in injecting high temperature fluids into oil fields and more particularly to structure for closing the ends of the annular space between two coaxially-mounted pipes so as to prevent impurities from entering this annular space while allowing relative longitudinal movement of the two pipes. This invention is usable in any application wherein high temperature fluids must be passed through an insulated pipe string at high pressures.
2. Prior Art
The injection of high temperature fluids into oil bearing strata is a common operation. In one application, high temperature steam at pressures up to, for example, 120 atmospheres, is injected into oil bearing strata to increase the recovery of oil from that strata. Insulating the pipe through which the steam is injected increases injection efficiency by reducing heat loss from the steam. This lowers costs, and by lowering the temperature of this casing reduces the likelihood that the casing will crack. If the casing cracks, it must be replaced. This is expensive.
An insulated pipe for such use in an oil well is disclosed in U.S. Pat. No. 3,574,357. In this patent the inner pipe through which the high temperature fluid flows has its upper end flared outwardly to receive the lower end of the adjacent mating pipe in a sliding, telescopic relationship. The inner pipe is then wrapped in insulation which in turn is surrounded by an outer load bearing pipe. The use of the outer pipe as the load bearing and pressure containing pipe increases the cost of the pipe string because the larger, outer pipe is more expensive than the inner pipe and must be of sufficient size and strength to hold the weight of the pipe string in the oil well. Moreover, inner telescoping pipes are likely to bind and therefore not slide freely, or to leak at the high pressures used with steam injection (such as 120 BARs) thereby allowing steam to enter the insulation and degrade its performance.
U.S. Pat. No. 2,419,278 discloses another approach to the solution of this problem. This patent proposes connecting an inner, fluid-bearing pipe to an outer load-bearing pipe through a bellows pipe flange welded to the edges of both the inner and outer pipes across the end of the annular space. This bellows shaped annular disc or plate extends in a radial direction from the inner to the outer pipe, thus limiting the total amount of longitudinal motion (i.e., motion along the pipe length) of the fluid-carrying inner pipe. Moreover, should a leak occur in this bellows-shaped disc, high pressure fluid or oil can enter the annular region thereby contaminating the insulation thus reducing its insulating properties. Upon removal of the pipe from the hole, should the small leak in the bellows shaped disc be clogged, the high pressure within the annular region creates a dangerous risk of explosion. In addition, this particular structure has a relatively limited ability to compensate for the different longitudinal expansions of the inner and outer pipes caused by the different temperatures at which these pipes operate. Thus only a limited longitudinal expansion of the inner pipe is allowed before the inner pipes in two directly adjacent sections come into contact and are unable to further longitudinally expand. Furthermore, experience has shown that the welds in pipes carrying high pressure fluids in corrosive environments such as a typical oil well are more likely to fail due to stress fatigue in the crystal-lographically altered pipe material next to the weld. Thus the solution in this patent is not suitable for the extremely high temperature and pressure operations now carried out in some oil fields.
A similar problem arises in two heat exchangers wherein fluid passing through tubes is changed in temperature by fluid passing on the outside of the tubes. When it is desired to prevent any mixing of the two fluids but at the same time allow for different longitudinal expansions of the tubes, an annular-shaped bellows is used to mount one end of the tubes to a stationary support structure. Such an annular-shaped bellows is disclosed for example in Belgian Pat. No. 569,050, patented on Jan. 2, 1959.
U.S. Pat. No. 3,693,665 issued Sept. 26, 1972, discusses the use of concentrically mounted pipes for the transport of cold liquids. In describing the prior art this patent states on Column 1, lines 58-63, "In known pipe lines of the above type, the shrinkage of the inner pipe is absorbed without great stresses by using bellows between successive inner pipes, or by providing the inner pipe with corrugations for part or all of its length." The patent then explains that pipelines with this structure have various disadvantages. In particular, the patent explains that "Bellows are weak points in a pipe line and repair or replacement is often difficult or impossible, especially in the case of submerged pipelines. Besides, bellows or corrugations have the disadvantage of causing extra flow resistance. Moreover, a corrugated inner pipe can only be produced by one of the few specialized manufacturers." Accordingly, the '665 patent disclosed a structure which allegedly overcame the disadvantages of a bellows-shaped inner pipe.
Other patents disclosing insulated pipes include U.S. Pat. Nos. 3,032,070, 3,068,026, 3,388,724, 3,511,282, 3,563,572, 3,608,640, 3,654,691, 4,025,091, 3,850,714, 4,139,024, 3,693,665, and 4,130,301.